Fix C standard inconsistencies

[tobil4sk]

Enables standards conformance in Visual Studio solution, and fixes violating code found in dx12.cpp (which was inconsistent with directx.cpp anyway). Also makes C standard consistent between build systems (all now use C11, whereas in VS this was not set and thus defaulted to "Legacy MSVC" standard).

Here the standards conformance option is explained: https://learn.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170